“Magnetic ranging” is a term given to determining the relative location of magnetic sources and magnetic detectors. It is known to apply magnetic ranging for guiding the drilling of one borehole in subterranean formations relative to another previously-drilled borehole.
The use of DC or low frequency magnetic fields for ranging is especially applicable in the subterranean environment because, unlike high frequency electromagnetic and acoustic methods, the fields are usually not affected by the earth formations. In most earth formations the magnetic permeability, which affects low frequency or DC magnetic fields, is usually constant and nearly equal to that of free space, whereas the resistivity and speed of sound which affect higher frequency electromagnetics and acoustics vary widely. A survey of magnetic ranging techniques and their use, included by reference in this patent, is in a paper by Grills et al. “Magnetic Ranging Technologies for Drilling Steam Assisted Gravity Drainage Well Pairs and Unique Well Geometries—A comparison of Technologies” SPE/Petroleum Society of CIM/CHOA 79005, Calgary, Alberta, Canada, 4-7 Nov. 2002 (“Grills”). Particular prior art applications of this technology, also included in this patent by reference, are described in U.S. Pat. No. 4,710,708, titled “Method and Apparatus Employing Received Independent Magnetic Field Components of a Transmitted Alternating Magnetic Field for Determining Location”, issued to Rorden et al. on Dec. 1, 1987, in CA Patent 2,147,610 (“Rorden”), in U.S. Reissue Pat. Re 36,569 of U.S. Pat. No. 5,485,089, titled “Method and Apparatus for Measuring Distance and Direction by Movable Magnetic Field Source” issued to Arthur Kuckes on Feb. 15, 2000 (“Kuckes”), and in US Patent Application No. 2009/0308657, titled “Magnetic Ranging And Controlled Earth Borehole Drilling” to Clark et al. (“Clark”).
These references describe methods using low frequency, elongated solenoid transmitter source(s) and three-axis magnetic field detector(s) for subterranean ranging. As described in the Grills reference above a particular use of magnetic ranging is to very accurately guide the drilling of a second borehole a specified distance and direction from a first or target borehole. A number of error sources are inherent in the prior art.
A major problem encountered in the prior art is the delay introduced during measurement of the magnetic field generated by energisation of solenoid sources. Delay in obtaining measurements impacts the ability to make course corrections. As Applicant understands it, in a conventional magnetic ranging method described in Kuckes and Clark, the measurement steps are as follows: the drilling is stopped and the solenoid sources are energised from the surface by passing either a DC current (Kuckes) or an AC current (Clark). The MWD including a three-axis magnetic field detector or three-axis magnetometer detects that drilling has stopped and activates the detector. The detector waits a first delay before measuring the various components of the magnetic field. This is because the detector is not aware whether or when the solenoid sources have been triggered and therefore whether the field being measured by the detector is the field generated by the energisation of the solenoid sources or other magnetic sources. In order to confirm whether the measured value relates to the magnetic field generated by the energisation of the solenoid sources, the detector compares the measured value with a reference value stored therein and which distinguishes solenoid activity from non-solenoid activity. If the measured value does not match the reference value, the MWD waits again for a pre-set period of time before taking another measurement. The whole process is repeated till the measured value satisfies the reference value. This verification introduces further delays in the measurement process. If the measured value matches the reference value, the measured value is typically transmitted to the surface through the drilling mud by resuming drilling as described in Kuckes. In Clark the measured value is transmitted to the surface via electromagnetic telemetry. Drilling need not be resumed to transmit the measured value to the surface in Clark. If, at the surface, it is determined that the measured value cannot be used to chart a corrective course, the whole process has to be repeated.
Uncertainty as to whether the field being measured by the detector is one that is created by the magnetic sources results in the first delay and the verification process described in the foregoing paragraph results in the second delay. These delays in the measurement process slow down the progress of drilling and can affect the accuracy and tortuosity of the second borehole.
Another problem encountered in Kuckes and Clark is when the MWD erroneously detects the state of the pumps and tool rotation. The survey can be started only when the tool is not rotating and the pumps are off. This state is normally is determined by the MWD tool by detecting small vibrations due to the mud flow in the pipe. This is subject to error. False detection of the state of the pumps by the MWD can result in a survey not being started at all or recorded data or values not being transmitted to the surface. Start of the survey is solely dependent on the MWD detecting the state of the pumps. If the MWD does not recognize pumps are off, it does not activate the detector for staring the survey. In this case when the pumps are turned on again the MWD would not begin pulsing data. The surface operator will then recognize that no pulses are being received and restart the measurement sequence by turning the pumps off. When the pumps have been off, sometimes the MWD tool also does not recognize the pumps being turned on once again and so does not send any data at all. This uncertainty results in further delays.
Fields generated or created due to the energisation of the solenoids fall off very quickly. Therefore, it is essential that the fields be measured as soon as they are created.
It is difficult to hold the tool completely still for a prolonged duration of time. A slight movement of the tool during the survey results in erroneous values being recorded. In Kuckes, for energisation of the solenoid current is passed in each direction for 30 seconds. Current is passed for 30 seconds in each direction because, the MWD waits a first delay, as explained above, before starting to record or measure. It is very difficult to hold the tool completely still for 60 seconds and the chances of getting a correct reading in Kuckes is, therefore, difficult.
In the prior art, errors can also be made when the boreholes are assumed to be parallel. While this assumption significantly simplifies the data reduction, the assumption can also result in convergence or divergence of the second and first boreholes. If the boreholes are assumed to be parallel, only distance and direction of the first borehole relative to the second borehole in a plane perpendicular to the second borehole are calculated, and not the direction or heading of the second borehole relative to the first borehole. Convergence or divergence of the second borehole from the first borehole cannot be measured until the next distance and direction measurement is taken, whereas if the heading is also measured, the convergence or divergence of the second borehole is known after the first measurement is taken. By measuring heading, a delay until a subsequent measurement is taken is avoided. Delays in determining convergence or divergence can lead to larger amplitude variations (errors) in the distance and direction to the other borehole by delaying the corrective action taken in the drilling of the second borehole.
Also known in the prior art is to make two measurements so that distance and direction may be estimated without knowledge of the dipole strength. In the two measurement method the solenoid is moved from the first position to the second position using a wireline from the surface. However the accuracy of the two measurement method depends on how accurately the distance between the first position and the second position is known. Since the solenoid is deployed in the borehole on a wireline the distance measurement is done from the surface and is subject to error factors such as wireline stretch and encoder wheel slippage. These errors directly affect the calculation distance from the first borehole to the second borehole.
Another error source in the prior art methods is reliance upon an assumption that the solenoid source is represented by a point dipole. If the boreholes are close enough together such that the length of the solenoid is a significant portion of the distance between boreholes the assumption that the solenoid is a point dipole and has no length causes errors especially in the measurement of field strength and distance between boreholes.
Another prior art error is caused by not determining the dipole strength at each measurement position. This method involves measuring the dipole strength at one position and expecting the solenoid dipole moment to remain the same strength at subsequent measurements. If the solenoid is in a cased borehole, variability in the type of casing will cause variable unknown attenuation of the magnetic field resulting in errors in distance calculations.
Another source of error in the prior art is the location of the magnetic poles at either end of the solenoid. When a prior art solenoid is deployed inside casing there is a “smearing” of the exact location of the solenoid pole ends. Not knowing the exact location of the magnetic source poles can cause large errors when the second borehole is being drilled within about 5 meters of the first borehole.
Considering all of the above error sources in the prior art, there is a need for a more accurate solution to the magnetic ranging problem. The present invention presents a more accurate apparatus and method which eliminates these error sources.